The discovery of superconductivity at temperatures above 77 K, the boiling point of liquid nitrogen, in oxides such as YBa.sub.2 Cu.sub.3 O.sub.7-x has stimulated a great deal of work over the past four years. Examples of other high temperature superconducting copper oxides which have been discovered during this period are the various Bi-based copper oxides , the various Tl-based copper oxides, the mixed Bi-Tl-based copper oxides and these superconducting oxides substituted with Pb and other substituents.
Much of the work relating to the use of these high temperature superconductors (HTSC) in microelectronic applications has been focused on the growth of high quality thin films. At microwave frequencies, 1-100 GHz, high quality superconducting thin films can have significantly lower values of surface resistance than copper or gold films. This low surface resistance is important for making high-performance thin film microwave circuits such as filters, resonators, and delay lines.
HTSC thin-films have been prepared by a number of deposition methods including evaporation, sputtering, laser ablation, and metallo-organic chemical vapor deposition (MOCVD), see, for example, R. W. Simon, Solid State Technology, p. 141, September, 1989 and J. Talvacchio et al., SPIE Proceedings Vol. 1292, published by SPIE Bellingham, Wash., (1990). In all of the thin film deposition processes for HTSC oxide materials, the substrate temperature is a critical parameter in determining the film quality. When the temperature of the substrate onto which the thin film of YBa.sub.2 Cu.sub.3 O.sub.7-x is being deposited is low, i.e., less than about 400.degree. C. the resulting film is amorphous and insulating. A high-temperature post-deposition anneal at 800.degree.-900.degree. C. in oxygen is required to convert the amorphous precursor film into a superconducting film with the correct crystal structure. This two-step process is referred to as a "post-anneal" process. However, under proper deposition conditions when a thin-film of YBa.sub.2 Cu.sub.3 O.sub.7-x is deposited on a hot substrate, i.e., one at a temperature of about 550.degree.-750.degree. C., the resulting thin-film is crystalline and superconducting directly as deposited. This process is referred to as an "in-situ" process. Films produced by an in-situ process have less surface roughness and generally superior properties when compared to films from a post anneal process. The in-situ deposition process has the added advantage of lower processing temperature requirements which makes the process compatible with a wider variety of substrate materials. In-situ processes based on laser ablation and sputtering have been found to produce high quality thin films of YBa.sub.2 Cu.sub.3 O.sub.7-x on various substrates.
All in-situ processes require a method of heating substrates uniformly and reproducibly to the required deposition temperature. At the present time, the best methods for Y.sub.1 Ba.sub.2 Cu.sub.3 O.sub.7-x thin films involve direct thermal and mechanical anchoring of the substrates to a heated block as discussed in R. W. Simon, SPIE Proceedings Vol. 1187, p 2, edited by T. Venkatesan, published by SPIE Bellingham, Wash., (1989). The heated block must be made of a material that is compatible with an oxygen atmosphere and has good thermal conductivity such as nickel. Simple mechanical clamping of the substrates to the block has proven to be unreliable and is not particularly useful for large area substrates because of substrate breakage. Good thermal contact between the substrate and the block is usually provided by a compound with high thermal conductivity such as silver paste. However, this introduces the complication of removing the paste from the back of the substrate usually by polishing or etching. In addition, the direct thermal contact method is not compatible with the deposition of an in-situ film on both sides of the substrate. The thermal contacting compound can interact with the film on the back surface of the substrate and the compound is difficult to remove without damaging the film underneath.
Yoshida et al., U.S. Pat. No. 5,206,216, disclose a process for forming a superconducting oxide film on a tape-type long base material. The base material is translated along its longitudinal direction and laser ablation is used to deposit the oxide film onto the moving base material. The base material is heated by radiant heat during the deposition. It is reported that it is possible to form a high-quality film only on a region of about 2 cm.sup.2 by laser deposition and therefore a restrictive mask is used to limit the area of deposition in order to form such a high-quality film.
The ability to deposit high quality superconducting thin films on both sides of a substrate is important for certain microwave circuit applications in order to improve performance and reliability. The film on one side of the substrate would serve as the ground plane while the film on the other side would be patterned into a microwave circuit. This invention provides a non-contact method of heating large area substrates to elevated temperatures for the purpose of depositing crystalline thin films of compounds, particularly films of high temperature superconducting oxides, requiring elevated growth temperatures on one or both sides of the substrate.